I have done it. After almost 17 full years of trying and experiencing at least three nightmares because of this game, I have finally, finally beaten Space Hulk and its main campaign.
Here's the problem I was having. During the mid-to-late 90s or thereabouts, I'd gotten brave enough to play said campaign and had eventually worked my way up to this mission, the third from the last.
Amongst other things---taking out any Genestealers in my way, and what not---I was supposed to retrieve an object (in this case, a scroll) and take it with me to the finishing point. I searched through the entire level, from top to bottom, even double-checking places that I'd already been to, and guess what? ...I couldn't find the stinkin' scroll! You can probably imagine my frustration.
Several times over the years I checked around online to see if I could find a guide, but there weren't any. Finally last night, I tried again---and this time I found one. When I checked it out, I found out why I could never find the scroll I was supposed to get: the stupid thing was hidden up near a batch of four "genebanks" (the Genestealers' spawning points) in the upper right portion of the level, a place that no sane person would dare to go, myself included. ...Yeah.
Compared to that fiasco, the latter two scenarios (one being a gigantic showdown between my squads and the Patriarch's forces, and the other a search mission) were cakewalks. I'm glad to finally be able to put this game behind me; hopefully I won't have those nightmares anymore.